Upgrading oil by in situ combustion



July 25, 1967 R. A. MORSE UPGRADING OIL BY IN SITU COMBUSTION Filed Dec. $50, 1964 MAJ MIVEN 70R.

RICHARD A. MORSE United States Patent 3,332,489 UPGRADING OIL BY IN SITU COMBUSTION Richard A. Morse, Barcelona, Venezuela, assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Dec. 30, 1964, Ser. No. 422,370 6 Claims. (Cl. 166--11) ABSTRACT OF THE DISCLOSURE A method of injecting oxidizing gas at a high rate into only the bottom of an oil-bearing formation, burning out in situ the upper portion of the formation, reducing the rate of the gas injection to stabilize the combustion front and vaporize an upgraded oil product, transporting the vaporized product through the burned out upper portion of the formation, through perforations adjacent only to the top of the formation and into a remote output Well and producing to the surface the fluids entering the output well.

This invention relates to the upgrading of crude oil in an underground reservoir and to the production of the upgraded product. The invention particularly relates to an in situ combustion process which is carried out for the maximum recovery of up-graded product.

Conventional or primary recovery techniques for recovering and producing oil in general produce between ten and thirty percent of the oil originally occuring in the underground reservoir. Many supplemental or secondary recovery techniques have been suggested such as waterfiooding, gas drive, thermal recovery, etc., and various of these have been used where they have been operationally and economically justified. The thermal methods of recovery include the injection of heated liquids or gases into the oil bearing formation and in situ combustion.

The conventional forward drive in situ combustion process is employed as a secondary recovery technique for the production of crude oil in situations where the reservoir characteristics, and crude oil properties economically justify this recovery approach. Air is injected into an input well and combustion is initiated by one of many accepted methods. It is hoped as an optimum result that the zone of combustion move as a radial front from the input well and drive the reservoir oil ahead of it to the production well. However, it frequently happens that the combustion gases and oil become segregated with the gases riding over the oil and channeling through the top of the formation as a result of the difference in densities and the increased fiowability of the heated oil. This overriding flame front prematurely breaks through into the production well prior to a substantial production of the oil. Once this breakthrough has occurred the continuation of air input will result in the production of an increasing amount of unreacted oxygen in the output well with a decrease in oil production. The occurrence of appreciable quantities of free oxygen in the output well in the presence of hydrocarbon materials at elevated temperatures poses a very real fire hazard. In addition the reduction in oil production as well as oxygen usage renders the process economically unsuitable after breakthrough has occurred.

Efiorts have been directed towards solving the problems introduced by this tendency of the reservoir to become stratified by the oxidizing gas overriding the oil. In one instance it has been suggested that a combustible hydrocarbon be injected into the top of the formation adjacent the input well to insure complete reaction of all of the oxygen in the combustion supporting gas. While the reservoir oil which is burned in in situ combustion 3,332,489 Patented July 25, 1967 does not represent an out-of-pocket expense to the operator, the fuel injected by this process represents a significant expense which may well render the recovery economically unfeasible. Another proposal suggests the separate recovery of the segregated crude oil and the overriding gas during combustion in order to keep hot oxidizing gas away from the oil product, but this involves a concomitant low oxygen utilization.

Some crude oils are of such low quality and high viscosity that they are produced only with difficulty at a substantially increased expense over light crudes. And once they are brought to the surface they must be prerefined to reduce asphaltic constituents and inorganic catalyst poisons at a cost amounting to as much as fifty percent of the well head price of the oil in order to put them in condition for conventional refining. It would be economically desirable if such an oil could be pretreated in the reservoir and produced as a prerefined upgraded oil.

Upgrading is a relative term which is used to indicate an increase in both quality and value. The upgraded oil recovery from the reservoir will contain a greater proportion of the move valuable lower boiling distillate material and a smaller amount of the less desired high boiling and asphaltic fractions than the virgin oil and may contain only distillate products. Since in general the lower boiling liquid fractions are the most desired, the relative increase in API gravity serves as a rough indication of the degree of upgrading.

I have discovered that in situ combustion can be carried out within the reservoir in a manner which will involve a high recovery of upgraded products in the output well. In my process the inherent tendency of the combustion gases and oil to segregate with the gases overriding the oil, which hitherto has been considered to be a serious impediment to the successful execution of conventional forward combustion, is advantageously utilized for the modification and production of the reservoir oil into an upgraded form. Perforation of the input well at the bottom of the formation and the production well at the top of the formation is an essential feature of my invention. A burned out, override zone of high permeability above the reservoir oil is first produced and is then utilized in conjunction with in situ combustion of reservoir oil under specific conditions of operation for the upgrading process. The upgraded product results from the thermal distillation of the lower boiling fractions together with the thermal cracking of the high boiling fractions and asphaltic materials in the presence of an oxidizing gas. The resulting combustion gases sweep through the override Zone of increased permeability to the production well carrying the upgraded fluid with them with a minimum encroachment of reservoir oil into the production well. Thus, I have discovered that the tendency of the combustion gases to override the reservoir oil can be utilized by appropriate adjustment of operating procedures and conditions to produce an upgraded product. Although any combustion supporting gas which is generally usable in in situ combustion such as oxygen, oxygen enriched air, and air is usable in carrying out my process, I prefer to use air due to inherent economy and my invention will be discussed with air as the oxidizing gas.

When a reservoir oil is to be produced as an upgraded product by my process, it is advantageous for economic reasons to produce the permeable override zone as quickly as possible with the retention of a sizeable amount of segregated oil in the reservoir for subsequent upgrading. Air is forced into the bottom of the formation through the input perforation at a high pressure with the concurrent initiation of combustion by self-ignition or by the use of a conventional igniter. Forward combustion is maintained at a high rate and override and early channeling towards the output perforation is enhanced by this high pressure injection. When breakthrough of the combustion front is imminent, air injection is substantially reduced in accordance with certain conditions to be described to maintain a relatively stationary combustion zone with the continuous production of the desired upgraded product.

Impending breakthrough of the combustion front is determined by temperature monitoring the output \V ll for a rapid rise in temperature or by analysis of the combustion gases for indication of a sharp rise in oxygen content. Unmodified reservoir oil is produced prior to the immediate approach of the flame front. This is followed by the production of hot volatilized hydrocarbons for a rela tively short period of time immediately preceding thermal breakthrough which serves also as advance warning of impending breakthrough. With this information on the imminency of breakthrough as indicated, the air flux is reduced and the well is put into production of the upgraded product. In this second stage of reduced air flow the operation is conducted under conditions that will result in the extinction of the combustion front in that portion of the formation closer to the output well and the continuation of a stable combustion zone in that portion of the formation closer to the input well. This second stage of operation is further characterized by the continuous production of a maximum of upgraded product with a minimum contamination with virgin oil, by a substantially complete oxygen utilization, and by a constant elevated temperature in the output well. The output is monitored as to temperature and oxygen content and these are controlled by the rate of air injection. An increase in air flux will increase the output well temperature and oxygen content and a decrease in air flux will decrease both.

In producing an upgraded product by this process it is desirable to obtain a sample of the specific reservoir crude to be treated and subject it to distillation and thermal cracking in the laboratory in simulation of underground conditions. This upgraded product is representative of the product to be obtained from the production well by optimum operation of my invention and may be used as a standard in evaluating and controlling the operation of my combined in situ combustion-upgrading process.

If a product containing a substantial amount of virgin oil is obtained in operating my process, it is likely that air injection is at a rate which is sufficient to drive reservoir oil out in admixture with the upgraded material. A reduction in air pressure should remedy this. My process is operated to satisfy a number of objectives as previously stated with essential control residing in the rate of air injection. I have ascertained that the process can be effectively operated by observing these variables being monitored and controlling the air injection in response thereto. The optimum production of distillate product should result if air is injected at a rate which will maintain the pressure differential within the formation between about one and two times the gravity head of the reservoir oil as determined by the difference in elevation between the points of air injection and product recovery. Since a large portion of the pressure differential is involved in overcoming the well bore resistance, the overall pressure differential as measured between the input and output wells will be from about one to ten times the gravity head in order to provide the requisite differential within the formation.

The drawing diagrammatically illustrates an oil bearing formation which is being operated in the second stage of reduced air injection for the production of the upgraded product. The oil bearing formation lies between impervious rock formations 11 and 12. Casing 13 of input well 14 extends completely through the oil formation down into contact with rock formation 12. In like manner casing 15 of the production well 16 extends through to rock layer 12. The input well casing has been perforated at 20 adjacent to the bottom of the casing and the production well casing has been perforated at 21 immediately below the upper boundary of the oil producing formation. It is preferred in sloping formations that the input wells be down-structure and the production wells be up-structure as shown.

In the first stage of operation, air or other suitable oxidizing gas is injected into input well 14 at a high pressure. Combustion will be self-initiated or if desired combustion may be initiated by one of the many well known ignition techniques. At this high pressure combustion will be relatively rapid with the combustion front riding over the oil bank and producing a burned out zone 22 of increased permeability in the upper portion of the reservoir. Virgin oil will be forced out of the reservoir into the production well ahead of the combustion front prior to breakthrough of the combustion front into the production well. The temperature of the production well is monitored and when a rapid temperature rise above the reservoir temperature is observed air injection is reduced and the reser voir is operated under conditions for maximum upgrading which is the stage of operation illustrated in the drawing.

With the reduced air input in this second stage of operation the combustion front extinguishes in the portion of the reservoir closer to the output well due to an insufiicient supply of oxygen and becomes localized in a zone 24 indicated by cross-hatching in the drawing in that portion of the reservoir closer to the input well 14. This combustion zone 24 will become stabilized at the constant, reduced rate of air injection and will operate at a temperature ranging from 600 to 1600 F. As a result of this elevated temperature the lower boiling more volatile components will be distilled out of the oil and will be carried by the combustion gases through the more permeable burned out zone 22 into the production well. In addition the high temperatures in the combustion zone will crack the higher boiling and asphaltic components into lower boiling upgraded components with the concurrent deposion of combustion supporting coke onto the formation particles in combustion zone 24. Thus, under optimum operation the product is a mixture of the low boiling fraction naturally present in the virgin crude and the low boiling fraction obtained by cracking the less volatile substances.

Oil from oil bank 25 will flow towards the combustion zone as indicated by arrows 26 as a result of gravity flow and aided by the gradual heating of the reservoir. The injected air will prevent the encroachment of this reservoir oil into the input perforation 20 and will keep the combustion zone out of immediate contact with the input well casing. Under stabilized steady state operation the combustion zone will be stationary with the injected air balancing the hydrostatic oil pressure. As the combustion gases and vaporized hydrocarbons travel from the combustion zone 24 to the output perforation 21 they are exposed to a gradual cooling. The formation adjacent output perforation 21 is maintained at a relatively stable elevated temperature ranging from 300 to 700 R, which is substantially cooler than the combustion zone. As the combustion gases travel through burned out zone 22 this cooling effect will cause the condensation of some of the higher boiling volatilized components. A portion of these condensed hydrocarbons will be forced out to the production well by the stream of combustion gases and the remainder Will move downwardly to the oil bank by gravity flow. This latter distillate material will co-mingle with the upper layer of the oil bank causing a decrease in its viscosity as a result of the mixing effect and by temperature elevation. This less viscous layer will tend to flow back towards the combustion zone as indicated by arrows 28 and supply reservoir oil to the upper portion of the combustion zone.

Injection of air and upgrading combustion is continued. Air injection is maintained relatively constant for maximum upgrading according to predetermined conditions. In this state of operation the oxygen content of the output gas will range from about one percent or less to about five percent and it is preferred that it be as low as possible. Excessive air flow into the reservoir will be indicated by an increase in oxygen content of the output well of over five percent or an increase in the temperature of the output well above about 600 to 700 F. or by the production of a substantial quantity of reservoir crude into the output well, or a combination of any of these factors. In this situation the air injection is reduced to obtain a balance of these factors in accordance with the predetermined goals. As the reservoir becomes depleted, it may be necessary to recomplete the production wells, perforating at lower and lower intervals as the oil level is reduced, keeping the perforation above the oil level at all times. It is also possible to achieve the same effect under specific reservoir conditions by slightly increasing the rate of air injection.

Since this process involves the injection of air at a moderate rate it inherently involves production at a moderate rate. In order to obtain the best economic advantage out of this process it is desired to operate at as high a rate of air injection as is possible without exceeding these predetermined conditions. I have discovered that operation to produce upgraded oil in accordance with my in vention usually involves air injection at a rate at which the differential pressure as measured between the input well and the output well is from about one to about ten times the gravity head of the oil in the reservoir as determined by the difference in elevation of the input perforation and the output perforation and is subject to the maintenance of the other operating conditions as noted.

This process has particular suitability with respect to poor quality crudes such as those havingan API gravity of about 5 to about 25. It may be used with those reservoir crudes which are amenable to in situ combustion and which may be economically upgraded by this method. It is of particular economic value where the reservoir oil must be pretreated or prerefined in order that it can be utilized in a conventional refinery. The well head price of this pretreated product is substantially increased over the well head price of the virgin crude oil. Not only is the product substantially upgraded as measured by API gravity but it is also greatly improved by a substantial reduction in the proportion of inorganic catalyst poisons including nitrogen, sulphur and heavy metals. Economic compensation is obtained not only in the substantial increase in the value of the upgraded product but also in the reduced air compressor costs as compared with ordinary in situ combustion. By passing air through the permeable burned out zone at moderate rates, relatively low air injection pressures are involved resulting in relatively low compressor costs per barrel of upgraded product. Operation at a reduced reservoir pressure is preferred because of the reduction in the boiling point of the hydrocarbons to be recovered and to reduce the required pressure at which the compressor is operated.

This process is now described in the recovery of an upgraded product. The natural drive of the reservoir has been exhausted and it now contains about 2000 bbl./ acre ft. of crude oil. The oil has an API gravity of 12 and a viscosity of 12,000 centistokes at the reservoir temperature of 80 F. The crude oil is upgraded in the laboratory to a product having an API gravity of 18 worth approximately $2.10 U.S. per barrel at the well head compared to $1.75 U.S. per barrel for the virgin crude. It is further ascertained that the crude oil supports in situ combustion. As a result of this'analysis it is decided that my process is economically advantageous for the recovery of additional product.

The producing formation is 100 feet thick with a pronounced slope. The input well is located down-structure and is perforated at a depth of 600 feet adjacent to the bottom of the pay sand. The output well is located upstructure, 100 feet from the input well, and is perforated adjacent the top of the pay sand between the depths of 450 to 500 feet. The gravity head of the oil is 42 p.s.i. between these perforations. Air is injected into the input well at the rate of 1.5 10 s.c.f./day for 30 days until a distinct temperature rise is noted in the output well. Up to this point virgin crude is produced and pumped from the output well. When the temperature reaches 500 F. the flow rate is reduced to maintain the pressure differential between the two wells at 2-00 p.s.i. After adjusting to this second phase of operation for ten days, steady state operation is attained. The temperature in the output well remains in the range of 300 to 700 F. and the oxygen in the output gases does not vary significantly from between 1 to 5 percent. A liquid product is pumped from the well and the volatiles are condensed out of the gases above ground. The combined product has an API gravity of 17.5 which represents substantially complete upgrading when compared with the optimum figure obtained from the laboratory analysis. The upgrading is determined to be successful and is continued for maximum recovery of the oil.

The invention was described in connection with the production of a burned out zone above the oil bank. It is possible to utilize my invention in conjunction with an existing burned out zone. For example a reservoir containing an uncased input well and uncased output well was subjected to conventional in situ combustion. An override, burned out zone was quickly produced with thermal breakthrough to the production well. Both wells are cased and perforated according to my process and air is injected as described herein for the production of an upgraded product. Although in general, it is desired that breakthrough of the flame front to the output well be avoided, it will not affect subsequent, successful operation of my process, if breakthrough does occur.

This invention was also described in connection with one input and one output well. However, it is intended and anticipated that it will be used in a variety of well patterns including conventional three spot, four spot, five spot and line drive patterns depending upon reservoir and formation characteristics. When multi-well operation is utilized the rate of air injection towards each production Well is controlled by controlling the pressure at the production well. Operation under this process can be continued for many months or longer for the substantially complete recovery of the oil as an upgraded product.

I claim:

1. A method of producing an upgraded oil from an un derground oil bearing formation by in situ combustion between an input well and an output well, each of said wells having casing installed and cemented through the oil-bearing formation, comprising perforating the input well only adjacent to the bottom of the formation, perforating the output well only adjacent to the top of the formation, injecting an oxidizing gas at a high pressure into the input well whereby in situ combustion takes place at a high rate with the combustion front overriding the main bulk of oil to form a burned out zone above the oil bank, monitoring the output well for the approach of the combustion front, reducing the rate of injection of oxidizing gas into the input well when the combustion front has reached the output well to a level at which a stable combustion zone is maintained in the formation nearer to the input well at a temperature in the range of 600 to 1600 F., continuing the injection of oxidizing gas at the reduced rate whereby an upgraded oil product is formed and vaporized in the combustion zone and carried by the combustion gases through the burned out zone to the output well, and recovering the upgraded product.

2. A method in accordance with claim 1 in which the oxidizing gas is air.

3. A method as set forth in claim 2 wherein said reduced air injection rate produces an injection Well pressure which is greater than the production well pressure by an amount in the range of 1 to 10 times the hydrostatic pressure produced by a column of the produced fluids whose height is equal to the difference between the elevation of the top of the oil sand at the producing well and the elevation of the top of the oil sand at the injection well.

4. In a method for producing oil by in situ combustion from an underground oil bearing formation penetrated by an input well and an output well, each having casing installed and cemented through the oil-bearing formation, in which a burned out zone exists through the formation between the wells above the oil bank, the improvement comprising perforating the input well only near the bottom of the formation, perforating the output well only near the top of the formation, injecting an oxidizing gas into the input well at a rate sufiicient to maintain a stable combustion zone in the formation without forcing virgin oil into the output well whereby an upgraded oil product is formed in the combustion zone and carried by the combustion gases through the burned out zone to the output well, and recovering the upgraded product.

5. A method in accordance with claim 4 in which the oxidizing gas is air.

6. A method of producing an upgraded oil from an underground oil-bearing formation by in situ combustion between an input well and an output well each having casing installed and cemented through the oil-bearing formation comprising:

perforating the input well adjacent only the bottom of the formation,

perforating the output well adjacent only the top of the formation,

injecting an oxidizing gas into the input well and through the oil-bearing formation at a rate sufiicient to form an override burned out zone above the oil bank between the injection well and the producing well,

thereafter reducing the injection rate of the oxidizing gas, to a reduced rate which maintains a substantially stable in situ combustion front in the oil formation at a temperature in the range of 600 F. to 1600 F. and substantially prevents liquid from flowing out of the input perforation whereby an upgraded oil product is formed and vaporized in the combustion zone and carried by the combustion gases through the burned out zone to the output well, and

recovering the upgraded product.

References Cited UNITED STATES PATENTS 25 CHARLES E. OCONNELL, Primary Examiner.

JAMES A. LEPPINK, Examiner, 

1. A METHOD OF PRODUCING AN UPGRADED OIL FROM AN UNDERGROUND OIL BEARING FORMATION BY IN SITU COMBUSTION BETWEEN AN INPUT WELL AND AN OUPUT WELL, EACH OF SAID WELLS HAVING CASING INSTALLED AND CEMENTED THROUGH THE OIL-BEARING FORMATION, COMPRISING PERFORATING THE INPUT WELL ONLY ADJACENT TO THE BOTTOM OF THE FORMATION, PERFORATING THE OUTPUT WELL ONLY ADJACENT TO THE TOP OF THE FORMATION, INJECTING AN OXIDIZING GAS AT A HIGH PRESSURE INTO THE INPUT WELL WHEREBY IN SITU COMBUSTION TAKES PLACE AT A HIGH RATE WITH THE COMBUSTION FRONT OVERRIDING THE MAIN BULK OF OIL TO FORM A BURNED OUT ZONE ABOVE THE OIL BANK, MONITORING THE OUTPUT WELL FOR THE APPROACH OF THE COMBUSTION FRONT, REDUCING THE RATE OF INJECTION OF OXIDIZING GAS INTO THE INPUT WELL WHEN THE COMBUSTION FRONT HAS REACHED THE OUTPUT WELL TO A LEVEL AT WHICH A STABLE COMBUSTION ZONE IS MAINTAINED IN THE FORMATION NEARER TO THE INPUT WELL AT A TEMPERATURE IN THE RANGE OF 600 ATO 1600* F., CONTINUING THE INJECTIN OF OXIDIZING GAS AT THE REDUCED RATE WHEREBY AN UPGRADED OIL PRODUCT IS FORMED AND VAPORIZED IN THE COMBUSTION ZONE AND CARRIED BY THE COMBUSTION GASES THROUGH THE BURNED OUT ZONE TO THE OUTPUT WELL, AND RECOVERING THE UPGRADED PRODUCT. 